netdev - Re: 8139cp: set ring address before enabling receiver

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Message-ID: <1353529135.2619.36.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 21 Nov 2012 20:18:55 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Woodhouse <dwmw2@...radead.org>
CC:	Jeff Garzik <jgarzik@...ox.com>, Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: Re: 8139cp: set ring address before enabling receiver

On Wed, 2012-11-21 at 19:51 +0000, David Woodhouse wrote:
> On Wed, 2012-11-21 at 13:12 -0500, Jeff Garzik wrote:
> > 
> > What sticks out at me from the commit message?
> > 
> > It was not tested on the famously quirky 8139 hardware at all.
> > 
> > While I have not looked at the 8139C+ data sheet in a while, sometimes 
> > the hardware _did_ have a strange init order.
> > 
> > As this works in a simulator but fails on real hardware, it seems like 
> > an obvious regression caused by an untested [on read hardware] patch.
> 
> The data sheet (v1.6, from http://realtek.info/pdf/rtl8139cp.pdf ) says
> in §6.33 (C+ Command Register):
>  "Enable C+ mode functions in C+CR register first,
>  => Enable transmit/receive in Command register (offset 37h),
>  => Configure other related registers (ex. Descriptor start address,
>     TCR, RCR, ...)."
>
> I understand the concern expressed in the offending commit message about
> DMA happening to invalid addresses, and I'll look at the data sheet
> harder to see when the DMA actually starts happening. But it definitely
> seems that our current code isn't doing what the data sheet says.
> 
> I wonder if I can find one of these lying around and stick it in a
> machine with an IOMMU...

You might be able to avoid disaster by doing:

1. Set MAC filter to drop everything
2. Enable RX DMA
3. Set RX DMA ring address
4. Set MAC filter according to current flags & multicast list

I'm assuming, knowing nothing about this particular hardware, that the
MAC filter register(s) will accept writes before RX DMA is enabled.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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